To provide a protection from the possibility of electrical shock, many appliances provide a connection between their exposed metallic portions and electrical ground. Should this connection fail for some reason, its measure of protection no longer remains available.
Most modern hospital beds include at least some electrical facilities, especially one or more motors to alter its configuration. Because of the possibly prolonged and intimate contact of a patient's skin with the bed frame, the connection to ground represents a particularly important safeguard. Accordingly, the safety of the bed increases where it includes an indication that its connection to ground remains intact.
A. P. Petzon et al. U.S. Pat. No. 3,716,876 incorporates, in the circuitry for a bed, a ground-indicating device. However, this simply amounts to a light bulb connected between ground and the side of the transformer receiving the a.c. power.
This device, consequently, suffers from several serious limitations. First, it lacks any ability to indicate when the a.c. voltage source has reversed polarity, with the "hot" wire appearing at the common terminal and vice versa.
Second, the common terminal in an a.c. voltage source should have a potential of ground, or zero volts. The circuit in Petzon et al. gives no alarm should the common line's voltage rise substantially above that.
Last, all current passing through the bulb, which generally remains lit, enters the ground system. The bulb from each bed would pass at least about 1 milliampere. In a hospital having several hundreds of beds, the current in the supposedly safe ground system could well exceed one-quarter of an ampere, an unacceptably high level.